Broiler breeders undergo intensive genetic selection and reassortment to improve progeny performance every year. High-performance broiler breeders normally face high challenges like stress, metabolic disease, etc., which lead to increased mortality and high economic losses in the flock. Calcium tetany is one of the important metabolic diseases in heavy-yielding broiler breeders. Although calcium tetany was first recognized as a significant cause of mortality over ten years ago, still very little information is available about this condition in scientific literature. Defects in calcium metabolism led to a decrease in calcium levels in the blood and result in muscle weakness & paralysis followed by death. The main etiology associated with the development of calcium tetany are poor pullet uniformity, early start of production, high body weight, high calcium in prelay feed or early start of laying feed, and small particle-sized calcium. Figure-1 shows different parameters involved in the development of calcium tetany.
CALCIUM REGULATION AND METABOLISM
Calcium andphosphorus regulation is critical for high-yielding broiler breeders as the shell gland will remove calcium from the blood to synthesizecalcium carbonate, whereas the excess phosphorus will be excreted. Roughly, 95% of the eggshell is calcium carbonate, 0.3% phosphorus, and 0.3% magnesium. Apart from eggshell formation, calcium has other important functions like the conduction of nerve impulses, and contraction of muscles and helps with circulatory and cardiac functions.Figure-2 represents the factors depending on the eggshell quality.
The formation of calcium carbonate in the shell gland of laying hens requires calcium both from the diet and from medullary bone to meet the amount of calcium needed for eggshell formation. Adequate levels and ratios of calcium and phosphorus are necessary fora laying hen diet for proper absorption of calcium and phosphorus. This is not only important for eggshell formation and bone turnover, but also for maintaining calcium and phosphorus homeostasis throughout the body and circulating blood supply.
Similarly, phosphorus is also used for cell growth and maintenance, muscle contractions, kidney function, regulation of heart rate, the transmission of nerve impulses, and maintaining blood pH, and is used as a co-factor for other minerals and vitamins like vitamin D, iodine, magnesium, zinc, etc.
A decrease in blood calcium during eggshell formation is regulated by the secretion of the parathyroid hormone, which increases calcium absorption through the intestine by stimulating 1,25-dihydroxy Vitamin D and also signals the kidney to reduce calcium excretion. In addition, an increase in estrogen levels during the prelay period helps for mobilization of calcium from bones without affecting bone strength by the formation of medullary bones.
Among various etiological factors which cause calcium tetany, the following are the key factors involved in the development of calcium tetany.
While feeding the parent stock, it’s essential to maintain good flock uniformity and to keep close to body-weight targets. As well, the key factors to be considered while formulating feed for the parent flock and assessing parent flock performance are feed composition, feed form, feeding management, and general management. Feed or diet should be balanced based onthe intake of digestible nutrients, level of feed intake, nutrient composition of feed, age of the bird, etc.
Calcium and phosphorus are the critical macro minerals for proper skeletal development, reproductive performance, shell quality, and other metabolic functions. High calcium diets for pullets and young hens that have not started production may negatively influence the metabolic systems needed to actively absorb and transport calcium for eggshell formation. The presence of excessive calcium levels before the onset of egg production may trigger a physiological response to eliminate the excess amount, which may or may not be reversible.
Calcium tetany is a problem associated with nutrition and feed management. Switching from prelay to breeder laying feed should be done at 5 to 10% egg production to supply the calcium that the hens need for egg production. When the breeder feed is provided early, and when egg production starts, the birds will not be able to absorb calcium efficiently. Providing breeder feed too late also results inthe depletion of bone calcium reserves. An adequate source and supply of calcium during lay is crucial to prevent the occurrence of calcium tetany.
Using large particles of calcium in the feed, which stays in the intestinal tract longer than a small particle of calcium, increases the time that it is available for absorption. Providing oyster shells or limestone grit in the afternoon hours is another good way to meet the hen’s calcium requirements. Breeder diets need to be formulated to nutrient levels advised by the primary breeder to provide optimal levels of calcium, phosphorus, vitamin D3, and important trace minerals.
2. MANAGEMENT (BODY WEIGHT & UNIFORMITY)
Management plays a vital role in the prevention or occurrence of calcium tetany.Flock uniformity& start of lightingarecrucial managemental practicesin preventing calcium tetany mortality. Poor flock uniformity leads to immature and overweight pullets duringphotostimulation. After photostimulation, the poorly conditioned and overweight pullets express more issues than pullets that are correctly conditioned. Moreover, flock uniformity is the main preliminary factor thatalso influences the point of sexual maturity of the birds. As well, poorly conditioned overweight pullets produce few or no eggswith high mortality during the laying stage. Early start of lighting and laying feed in the poor uniform flock will lead to ahigher incidence of calcium tetany.
3. STRESS AND BLOOD pH
The environment is very important to promote the good performance of birds. Stresses such as high humidity, improper running of fans, poor ventilation, inadequate water supply, overweight or over-fleshed birds, high bird densities, and poorly maintained equipment can cause poor performance. Metabolic activity and increased bird activity after feed intake createa stressful event that happens primarily at feeding time. The birds generate a lot of metabolic heat while eating and digesting. Metabolic disturbances that increase blood pH, called respiratory or metabolic alkalosis will affect calcium to change in the blood from the ionized (active) to the protein-bound (inactive) form. This metabolic stress and blood pH imbalance will disrupt calcium metabolismresulting in low bloodcalcium levels and predisposing hens to calcium tetany. It’s obvious that during early morning or after feeding, the flock shows muscle weakness and paralysis associated with calcium tetany.
4. ELECTROLYTE IMBALANCE
Electrolyte balance plays a vital role in maintaining the metabolic health of the chicken, including calcium homeostasis. Electrolyte balance will be affected by the disease of either organ – the intestinal tract or kidney, responsible for absorption or excretion, feed formulation, and water quality. Calcium absorption and excretion are closely linked to phosphorus. A significant impact on calcium regulation is seen during high or low levels of phosphorus in the diet or blood. Low chloride levels can affect the acid-base balance and increase blood pH leading to metabolic or respiratory alkalosis, changing the calcium to inactive form and resulting in calcium tetany and mortality.
5. INTESTINAL HEALTH
The bird relies on the diet to supply the calcium that it needs for eggshell formation, replenishment of bone stores, and metabolic processes. Calcium must be absorbed through the intestine to reach the bloodstream. Any factor that increases feed passage rate like poor intestinal health, or poor water quality like saline water with high pH will limit calcium absorption. The intestinal tract and kidneys are the two major organs responsible for the absorption and excretion of calcium through complex mechanisms and for maintaining electrolyte balance. Disturbance to these organs in the form of the disease, raw material quality, feed composition, feed formulation, or water contaminations will directly affect the electrolyte balance and in turn, result in metabolic diseases like calcium tetany.
CALCIUM TETANY IN CLOSED HOUSE BIRDS IN BATTERY CAGES
Two different broiler breeder flocks in different periods in a battery cage system of rearing in a closed house with controlled temperature, humidity, and ventilation suffered heavily with sudden high mortality at the early production phase of the bird life cycle. Feed intake and production improvement are normal with no major clinical signs.
Typical post-mortem findings include an active, congested ovary and oviduct (Figure.4), and a partially or fully formed egg in the oviduct. Mortality is more obvious during the early morning period and after feeding. These findings are noticed in a healthy bird with no leg problems or other lesions. Initially, mortality is suspected for different reasons like infectious diseases and managemental problems like sudden death syndrome. Later, based on the negligible response from phosphorus and potassium supplementation, and negative laboratory tests for infectious causes, mortality is suspected for calcium tetany.
Different reasons suspected for calcium tetany in the first breeder unit are a high calcium level of 2% in the pre-lay feed and high body weight.Whereas,the second breeder unit had high humidity inside the shed and poor flock uniformity fed with a standard level of 1.2% calcium in prelay feed and 3% calcium in layer feed. Nevertheless, even after correcting the etiology, mortality continued till peak production and mortality started to decline only after post-peak production. This etiology in both the breeder units was corrected in subsequent flocks and calcium tetany was prevented. For subsequent batches inthe second breeder unit, cautiously a gradual increase of calcium level from 1.2% to 2%wasfollowedduringthe initial laying period up to 30% production. Calcium levels were further increased from 2% to 3.1% in regular layer feed from 30% productionand calcium tetany was prevented.
SUGGESTIONS TO PREVENT CALCIUM TETANY
Maintain pullet body weight and uniformity as per breed standard recommendation of 85%
Avoid high calcium breeder feed up to 5% production and maintain a calcium level of 1.2% in prelay feed to 5% production
Maintain proper water quality by regular sanitation and acidification of drinking water at 1ml/4 liters of water and periodic cleaning of water tanks
Maintain intestinal health by using strong probiotics at 200 g per ton of feed
Proper maintenance of shed temperature, ventilation, and humidity to avoid metabolic disturbance and reduce the stress on the birds
Blood pH and electrolyte balance should be maintained with proper calcium and phosphorus homeostasis throughout the bird life cycle
Calcium tetany is a metabolic disorder, particularly in heavy-yielding broiler breeders. Calcium tetany is mainly caused by hypocalcemia of broiler breeder hens at the onset of lay to peak egg production.Birds die with or without any clinical symptoms and birds typically die in the morning to early afternoon hours. The distinctive necropsy findings include an active, congested ovary, with a partially or fully formed egg in the oviduct. Definitive diagnosis is hard as it involves testing blood calcium levels in affected birds that are difficult to find in the acute phase. Treatment of severe cases of tetany includes calcium supplementation in the form of oyster shell or large particle limestone at 2-5g per bird per day for three consecutive days followed by three days of rest. Continuous high levels of supplementation can negatively affect calcium regulation and the condition will be worsened. This program can be continued until the mortality is under control. The active form of Vitamin D can be used concurrently to increase the absorption of dietary calcium.
Maintaining standard body weight and pullet uniformity will reduce calcium tetany incidence. Avoid feeding high calcium breeder feed until 5% production. Incorporate large particle size calcium to prolong intestine passage time and to favor increased calcium absorption. Manage blood pH change and maintain electrolyte balance from time to time. Maintain good quality water and gut health to promote proper absorption of calcium and phosphorus from the intestine. Follow breed standards regarding feed shifting and nutrient composition to get better breeder performance.
by Dr. N. Jagadeesh and Dr. R. Chanthirasekaran, Kemin Industries